Socket programming-in-python

Sockets programming in Python
Develop networking applications using Python's basic sockets
features
Skill Level: Intermediate
M. Tim Jones (mtj@mtjones.com)
Senior Principal Software Engineer
Emulex Corp.
04 Oct 2005
This tutorial shows how to develop sockets-based networking applications using
Python. In this tutorial, you first learn a few Python basics and see why Python makes
a good network programming language. Then you move on to the basic sockets
features of Python, using a sample chat application as a guide, and look at several
other, high-level, classes that provide asynchronous communications.
Section 1. Before you start
About this tutorial
Python is a popular object-oriented scripting language with a simple syntax and a
large developer base. It is a general purpose language and can be used in a variety
of settings. It's also popular as a beginner's programming language, much like the
BASIC language of the 1970s.
This tutorial demonstrates the Python language with emphasis on networking
programming. I define the basic sockets features of Python in addition to some of
Python's other classes that provide asynchronous sockets. I also detail Python's
application-layer protocol classes, showing how to build Web clients, mail servers
and clients, and more.
© Copyright IBM Corporation 1994, 2006. All rights reserved. Page 1 of 25

I also demonstrate a simple chat server to illustrate the power of Python for sockets
applications.
You should have a basic understanding of the standard BSD Sockets API and some
experience with the GNU/Linux® environment. Some familiarity with object-oriented
concepts is also beneficial.
Prerequisites
This tutorial and the examples demonstrated in it rely on version 2.4 of Python. You
can download this version from the Python Web site (see Resources for a link). To
build the Python interpreter, you need the GNU C compiler (gcc) and the
configure/make utilities (which are part of any standard GNU/Linux distribution).
You should have a basic understanding of the standard BSD Sockets API and some
experience with the GNU/Linux environment. Some familiarity with object-oriented
concepts is also beneficial.
Section 2. Introducing Python
First, I'll provide a taste of Python.
What is Python?
Python is a general purpose object-oriented scripting language that can be applied
to a large variety of problems. It was created in the early 1990s at CWI in
Amsterdam and continues to evolve today under the Python Software Foundation.
Python is amazingly portable and can be found in almost all operating systems.
Python is interpreted and is easy to extend. You can extend Python by adding new
modules that include functions, variables, or types through compiled C or C++
functions.
You can also easily embed Python within C or C++ programs, allowing you to extend
an application with scripting capabilities.
One of the most useful aspects of Python is its massive number of extension
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modules. These modules provide standard functions such as string or list
processing, but there are also application-layer modules for video and image
processing, audio processing, and yes, networking.
A taste of Python
I'll give you a sense of what Python is all about.
As an interpreted language, it's easy to use Python to try out ideas and quickly
prototype software. Python programs can be interpreted as a whole or line by line.
You can test the following snippets of Python code by first running Python, then
typing each line one at a time. After Python is invoked, a prompt (>>>) appears to
allow you to type commands. Note that indentation is important in Python, so the
preceding spaces in a line must not be ignored:
Listing 1. Some Python samples to try out
# Open a file, read each line, and print it out
for line in open('file.txt'):
print line
# Create a file and write to it
file = open("test.txt", "w")
file.write("test linen")
file.close()
# Create a small dictionary of names and ages and manipulate
family = {'Megan': 13, 'Elise': 8, 'Marc': 6}
# results in 8
family['Elise']
# Remove the key/value pair
del family['Elise']
# Create a list and a function that doubles its input. Map the
# function to each of the elements of the list (creating a new
# list as a result).
arr = [1, 2, 3, 4, 5]
def double(x): return x*x
map(double, arr)
# Create a class, inherit by another, and then instantiate it and
# invoke its methods.
class Simple:
def __init__(self, name):
self.name = name
def hello(self):
print self.name+" says hi."
class Simple2(Simple):
ibm.com/developerWorks developerWorks®

def goodbye(self):
print self.name+" says goodbye."
me = Simple2("Tim")
me.hello()
me.goodbye()
Why use Python?
The number one reason to learn and use Python is its popularity. The size of its user
base and the growing number of applications built with Python make it a worthwhile
investment.
You find Python in several development areas -- it's used to build system utilities, as
a glue language for program integration, for Internet applications, and for rapid
prototyping.
Python also has some advantages over other scripting languages. It has a simple
syntax and is conceptually clear, making it easy to learn. Python is also easier and
more descriptive when using complex data structures (such as lists, dictionaries, and
tuples). Python can also extend languages and be extended by languages in turn.
I find that Python's syntax makes it more readable and maintainable than Perl but
less so than Ruby. Python's advantage over Ruby is the large number of libraries
and modules that are available. Using these, you can build feature-rich programs
with little custom code.
Python's use of indentation to identify block scope can be rather annoying, but its
simplicity tends to make up for this minor flaw.
Now, let's dig into sockets programming in Python.
Section 3. Python sockets modules
Basic Python sockets modules
Python offers two basic sockets modules. The first, Socket, provides the standard
BSD Sockets API. The second, SocketServer, provides a server-centric class that
simplifies the development of network servers. It does this in an asynchronous way
in which you can provide plug-in classes to do the application-specific jobs of the

server. Table 1 lists the classes and modules that this section covers.
Table 1. Python classes and modules
Class/module Description
Socket Low-level networking interface (per the BSD API)
SocketServer Provides classes that simplify the development of
network servers
Let's look at each of these modules to understand what they can do for you.
The Socket module
The Socket module provides the basic networking services with which UNIX®
programmers are most familiar (otherwise known as the BSD API). This module has
everything you need to build socket servers and clients.
The difference between this API and the standard C API is in its object orientation. In
C, a socket descriptor is retrieved from the socket call then used as a parameter to
the BSD API functions. In Python, the socket method returns a socket object to
which the socket methods can be applied. Table 2 provides some of the class
methods and Table 3 shows a subset of the instance methods.
Table 2. Class methods for the Socket module
Class method Description
Socket Low-level networking interface (per the BSD API)
socket.socket(family, type) Create and return a new socket object
socket.getfqdn(name) Convert a string quad dotted IP address to a fully
qualified domain name
socket.gethostbyname(hostname) Resolve a hostname to a string quad dotted IP
address
socket.fromfd(fd, family, type) Create a socket object from an existing file
descriptor
Table 3. Instance methods for the Socket module
Instance method Description
sock.bind( (adrs, port) ) Bind the socket to the address and port
sock.accept() Return a client socket (with peer address
information)
sock.listen(backlog) Place the socket into the listening state, able to

pend backlog outstanding connection requests
sock.connect( (adrs, port) ) Connect the socket to the defined host and port
sock.recv( buflen[, flags] ) Receive data from the socket, up to buflen
bytes
sock.recvfrom( buflen[, flags] ) Receive data from the socket, up to buflen
bytes, returning also the remote host and port
from which the data came
sock.send( data[, flags] ) Send the data through the socket
sock.sendto( data[, flags], addr ) Send the data through the socket
sock.close() Close the socket
sock.getsockopt( lvl, optname ) Get the value for the specified socket option
sock.setsockopt( lvl, optname, val ) Set the value for the specified socket option
The difference between a class method and an instance method is that instance
methods require a socket instance to be performed (returned from socket), where
class methods do not.
The SocketServer module
The SocketServer module is an interesting module that simplifies the
development of socket servers. A discussion of its use is far beyond the scope of
this tutorial, but I'll demonstrate its basic use and then refer you to the Resources
section for links to a more detailed discussion.
Consider the simple case shown in Listing 2. Here, I implement a simple "Hello
World" server which emits the message when a client connects. I first create a
request handler that inherits the SocketServer.StreamRequestHandler class.
I define a method called handle that handles the requests for the server. Everything
the server does must be handled within the context of this function (at the end, the
socket is closed). This process works in simple cases, but you can implement even
simple HTTP servers with this class. In the handle method, I emit my salutation and
then exit.
Now that the connection handler is ready, all that's left is to create the socket server.
I use the SocketServer.TCPServer class, providing the address and port
number (to which the server will be bound) and my request-handler method. The
result is a TCPServer object. Calling the serve_forever method starts the server
and makes it available for connections.
Listing 2. Implementing a simple server with the SocketServer module

import SocketServer
class hwRequestHandler( SocketServer.StreamRequestHandler ):
def handle( self ):
self.wfile.write("Hello World!n")
server = SocketServer.TCPServer( ("", 2525), hwRequestHandler )
server.serve_forever()
That's it! Python permits a number of variations on this theme, including
UDPServers and forking and threading servers. See the Resources section for links
to more details.
Section 4. Sockets programming in Python
In languages with sockets, the socket is universally the same -- it's a conduit
between the two applications that can communicate with one another.
Preliminaries
Whether you're writing a sockets application in Python, Perl, Ruby, Scheme, or any
other useful language (and by useful I mean languages that have a sockets
interface), the socket is universally the same. It's a conduit between the two
applications that can communicate with one another (either locally on a single
machine or between two machines in separate locations).
The difference with sockets programming in a language like Python is in the helper
classes and methods that can simplify sockets programming. In this section I'll
demonstrate the Python socket API. You can execute the Python interpreter with a
script or, if you execute Python by itself, you can interact with it one line at a time. In
this way, you can see the result of each method invoked.
The following example illustrates interacting with the Python interpreter. Here, I use
the socket class method gethostbyname to resolve a fully qualified domain name
(www.ibm.com) to a string quad-dotted IP address ('129.42.19.99'):
Listing 3. Using the socket API from the interpreter command line
[camus]$ python
Python 2.4 (#1, Feb 20 2005, 11:25:45)
[GCC 3.2.2 20030222 (Red Hat Linux 3.2.2-5)] on linux2
Type "help", "copyright", "credits" or "license" for more
information.
>>> import socket

>>> socket.gethostbyname('www.ibm.com')
'129.42.19.99'
>>>
After the socket module is imported, I invoke the gethostbyname class method to
resolve the domain name to an IP address.
Now, I'll discuss the basic socket methods and communicating through sockets.
Feel free to follow along with your Python interpreter.
Creating and destroying sockets
To create a new socket, you use the socket method of the socket class. This is a
class method because you don't yet have a socket object from which to apply the
methods. The socket method is similar to the BSD API, as demonstrated in the
creation of a stream (TCP) and datagram (UDP) socket:
Listing 4. Creating stream and datagram sockets
streamSock = socket.socket( socket.AF_INET, socket.SOCK_STREAM )
dgramSock = socket.socket( socket.AF_INET, socket.SOCK_DGRAM )
In each case, a socket object is returned. The AF_INET symbol -- argument one --
indicates that you're requesting an Internet Protocol (IP) socket, specifically IPv4.
The second argument is the transport protocol type (SOCK_STREAM for TCP sockets
and SOCK_DGRAM for UDP sockets). If your underlying operating system supports
IPv6, you can also specify socket.AF_INET6 to create an IPv6 socket.
To close a connected socket, you use the close method:
streamSock.close()
Finally, you can delete a socket with the del statement:
del streamSock
This statement permanently removes the socket object. Attempting to reference
the socket thereafter produces an error.
Socket addresses
An endpoint address for a socket is a tuple consisting of an interface address and a
port number. Because Python can represent tuples easily, the address and port are

represented as such. This illustrates an endpoint for interface address 192.168.1.1
and port 80:
( '192.168.1.1', 80 )
You can also use a fully qualified domain name here, such as:
( 'www.ibm.com', 25 )
This example is simple and certainly beats the sockaddr_in manipulation that's
necessary in C. The following discussion provides examples of addresses in Python.
Server sockets
Server sockets are typically those that expose a service on a network. Because
server and client sockets are created in different ways, I discuss them
independently.
After you create the socket, you use the bind method to bind an address to it, the
listen method to place it in the listening state, and finally the accept method to
accept a new client connection. This is demonstrated below:
Listing 5. Using server sockets
sock = socket.socket( socket.AF_INET, socket.SOCK_STREAM )
sock.bind( ('', 2525) )
sock.listen( 5 )
newsock, (remhost, remport) = sock.accept()
For this server, the address ('', 2525) is used which means that the wildcard is
used for the interface address (''), allowing incoming connections from any
interface on the host. You also bind to port number 2525.
Note here that the accept method returns not only the new socket object that
represents the client connection (newsock) but also an address tuple (the remote
address and port number of the peer end of the socket). Python's SocketServer
module can simplify this process even further, as demonstrated above.
You can also create datagram servers, but they are connectionless and therefore
have no associated accept method. The following example creates a datagram
server socket:
Listing 6. Creating a datagram server socket
sock = socket.socket( socket.AF_INET, socket.SOCK_DGRAM )
sock.bind( ('', 2525) )

The upcoming discussion of sockets I/O shows how I/O works for both stream and
datagram sockets.
Now, let's explore how a client creates a socket and connects it to a server.
Client sockets
The mechanisms for creating and connecting client sockets are similar to the setup
of server sockets. Upon creating a socket, an address is needed -- not to locally bind
the socket (as is the case with a server) but rather to identify where the socket
should attach. Say there's a server on a host with an interface IP address of
'192.168.1.1' and port 2525. The following code creates a new socket and connects
it to the defined server:
Listing 7. Creating a stream socket and connecting to the server
sock.connect( ('192.168.1.1', 2525) )
For datagram sockets, the process is a bit different. Recall that datagram sockets
are by nature disconnected. One way to the think about it is as follows: Whereas
stream sockets are pipes between two endpoints, datagram sockets are
message-based, able to communicate with multiple peers at the same time. Here's
an example of a datagram client.
Listing 8. Creating a datagram socket and connecting to the server
sock = socket.socket( socket.AF_INET, sock.sock_DGRAM )
sock.connect( ('192.168.1.1', 2525) )
What's different here is that even though I've used the connect method, there's no
real connection between the client and server. The connect here is a simplification
for later I/O. Typically in datagram sockets, you must provide the destination
information with the data that you want to send. By using connect, I've cached this
information with the client and send methods can occur much like stream socket
versions (no destination address is necessary). You can call connect again to
re-specify the target of the datagram client's messages.
Stream sockets I/O
Sending or receiving data through stream sockets is simple in Python. Several
methods exist to move data through a stream socket (such as send, recv, read,
and write).

This first example demonstrates a server and client for stream sockets. In this
demonstration, the server echoes whatever it receives from the client.
The echo stream server is presented in Listing 9. Upon creating a new stream
socket, an address is bound to it (accept connections from any interface and port
45000) and then the listen method is invoked to enable incoming connections.
The echo server then goes into a loop for client connections. The accept method is
called and blocks (that is, does not return) until a new client connects, at which point
the new client socket is returned along with address information for the remote
client. With this new client socket, I call recv to get a string from the peer, then write
this string back out to the socket. I then immediately close the socket.
Listing 9. Simple Python stream echo server
import socket
srvsock = socket.socket( socket.AF_INET, socket.SOCK_STREAM )
srvsock.bind( ('', 23000) )
srvsock.listen( 5 )
while 1:
clisock, (remhost, remport) = srvsock.accept()
str = clisock.recv(100)
clisock.send( str )
clisock.close()
Listing 10 shows the echo client that corresponds with the server in Listing 9. Upon
creating a new stream socket, the connect method is used to attach this socket to
the server. When connected (when the connect method returns), the client emits a
simple text message with the send method, then awaits the echo with the recv
method. The print statement is used to emit what's read. When this is done, the
close method is performed to close the socket.
Listing 10. Simple Python stream echo server
import socket
clisock = socket.socket( socket.AF_INET, socket.SOCK_STREAM )
clisock.connect( ('', 23000) )
clisock.send("Hello Worldn")
print clisock.recv(100)
clisock.close()
Datagram sockets I/O
Datagram sockets are disconnected by nature which means that communication
requires that a destination address be provided. Similarly, when a message is

received through a socket, the source of the data must be returned. The recvfrom
and sendto methods support the additional address information as you can see in
the datagram echo server and client implementations.
Listing 11 shows the datagram echo server. A socket is first created and then bound
to an address using the bind method. An infinite loop is then entered for serving
client requests. The recvfrom method receives a message from the datagram
socket and returns not only the message but also the address of the source of the
message. This information is then turned around with the sendto method to return
the message to the source.
Listing 11. Simple Python datagram echo server
import socket
dgramSock.bind( ('', 23000) )
while 1:
msg, (addr, port) = dgramSock.recvfrom( 100 )
dgramSock.sendto( msg, (addr, port) )
The datagram client is even simpler. After creating a datagram socket, I use the
sendto method to send a message to a specific address. (Remember: Datagrams
have no connection.) After sendto finishes, I await the echo response with recv,
then print it. Note that I don't use recvfrom here because I'm not interested in the
peer address information.
Listing 12. Simple Python datagram echo client
import socket
dgramSock.sendto( "Hello Worldn", ('', 23000) )
print dgramSock.recv( 100 )
dgramSock.close()
Socket options
Sockets default to a set of standard behaviors, but it's possible to alter the behavior
of a socket using options. You manipulate socket options with the setsockopt
method and capture them with the getsockopt method.
Using socket options is simple in Python, as demonstrated in Listing 13. In the first
example, I read the size of the socket send buffer. In the second example, I get the
value of the SO_REUSEADDR option (reuse the address within the TIME_WAIT
period) and then enable it.

Listing 13. Using socket options
# Get the size of the socket's send buffer
bufsize = sock.getsockopt( socket.SOL_SOCKET, socket.SO_SNDBUF )
# Get the state of the SO_REUSEADDR option
state = sock.getsockopt( socket.SOL_SOCKET, socket.SO_REUSEADDR )
# Enable the SO_REUSEADDR option
sock.setsockopt( socket.SOL_SOCKET, socket.SO_REUSEADDR, 1 )
The SO_REUSEADDR option is most often used in socket server development. You
can increase the socket send and receive buffers for greater performance, but given
that you're operating here in an interpreted scripting language, it may not provide
you with much benefit.
Asynchronous I/O
Python offers asynchronous I/O as part of the select module. This feature is
similar to the C select mechanism but has some simplifications. I'll first introduce
select and then show you how to use it in Python.
The select method allows you to multiplex events for several sockets and for
several different events. For example, you can instruct select to notify you when a
socket has data available, when it's possible to write data through a socket, and
when an error occurs on a socket; and you can perform these actions for many
sockets at the same time.
Where C works with bitmaps, Python uses lists to represent the descriptors to
monitor and also the return descriptors whose constraints are satisfied. Consider the
following example in which you await some input from standard input:
Listing 14. Awaiting input from stdin
rlist, wlist, elist = select.select( [sys.stdin], [], [] )
print sys.stdin.read()
The arguments passed to select are lists representing read events, write events,
and error events. The select method returns three lists containing the objects
whose events were satisfied (read, write, exception). In this example, upon return
rlist should be [sys.stdin], indicating that data are available to read on stdin.
This data are then read with the read method.
The select method also works on socket descriptors. In the following example (see
Listing 15), two client sockets are created and connected to a remote peer. The

select method is then used to identify which socket has data available for reading.
The data are then read and emitted to stdout.
Listing 15. Demonstrating the select method with multiple sockets
import socket
import select
sock1 = socket.socket( socket.AF_INET, socket.SOCK_STREAM )
sock2 = socket.socket( socket.AF_INET, socket.SOCK_STREAM )
sock1.connect( ('192.168.1.1', 25) )
sock2.connect( ('192.168.1.1', 25) )
while 1:
# Await a read event
rlist, wlist, elist = select.select( [sock1, sock2], [], [], 5 )
# Test for timeout
if [rlist, wlist, elist] == [ [], [], [] ]:
print "Five seconds elapsed.n"
else:
# Loop through each socket in rlist, read and print the available data
for sock in rlist:
print sock.recv( 100 )
Section 5. Building a Python chat server
A simple chat server
You've explored the basic networking APIs for Python; now you can put this
knowledge to use in a simple application. In this section, you'll build a simple chat
server. Using Telnet, clients can connect to your Python chat server and globally
communicate with one another. Messages submitted to the chat server are viewed
by others (in addition to management information, such as clients joining or leaving
the chat server). This model is shown graphically in Figure 1.
Figure 1. The chat server uses the select method to support an arbitrary
number of clients

An important requirement to place on your chat server is that it must be scalable.
The server must be able to support an arbitrary number of stream (TCP) clients.
To support an arbitrary number of clients, you use the select method to
asynchronously manage your client list. But you also use a feature of select for
your server socket. The read event of select determines when a client has data
available for reading, but it also can be used to determine when a server socket has
a new client trying to connect. You exploit this behavior to simplify the development
of the server.
Next, I'll explore the source of the Python chat server and identify the ways in which
Python helps simplify its implementation.
The ChatServer class
Let's start by looking at the Python chat server class and the __init__ method --
the constructor that's invoked when a new instance is created.
The class is made up of four methods. The run method is invoked to start the server
and permit client connections. The broadcast_string and
accept_new_connection methods are used internally in the class and will be
discussed shortly.
The __init__ method is a special method that's invoked when a new instance of
the class is created. Note that all methods take the self argument, a reference to
the class instance itself (much like the this parameter in C++). You'll see the self

parameter, part of all instance methods, used here to access instance variables.
The __init__ method creates three instance variables. The port is the port
number for the server (passed in the constructor). The srvsock is the socket object
for this instance, and descriptors is a list that contains each socket object for
the class. You use this list within the select method to identify the read event list.
Finally, Listing 16 shows the code for the __init__ method. After creating a stream
socket, the SO_REUSEADDR socket option is enabled so that the server can be
quickly restarted, if necessary. The wildcard address is bound with the defined port
number. Then the listen method is invoked to permit incoming connections. The
server socket is added to the descriptors list (the only element at present), but all
client sockets will be added as they arrive (see accept_new_connection). A
salutation is provided to stdout indicating that the server has started.
Listing 16. The ChatServer class with the init method
import socket
import select
class ChatServer:
def __init__( self, port ):
self.port = port;
self.srvsock = socket.socket( socket.AF_INET, socket.SOCK_STREAM )
self.srvsock.setsockopt( socket.SOL_SOCKET, socket.SO_REUSEADDR, 1 )
self.srvsock.bind( ("", port) )
self.srvsock.listen( 5 )
self.descriptors = [self.srvsock]
print 'ChatServer started on port %s' % port
def run( self ):
...
def broadcast_string( self, str, omit_sock ):
...
def accept_new_connection( self ):
...
The run method
The run method is the server loop for your chat server (see Listing 17). When
called, it enters an infinite loop, providing communication between connected clients.
The core of the server is the select method. I pass the descriptor list (which
contains all the server's sockets) as the read event list to select (and null lists for
write and exception). When a read event is detected, it's returned as sread. (I
ignore the swrite and sexc lists.) The sread list contains the socket objects that

will be serviced. I iterate through the sread list, checking each socket object found.
The first check in the iterator loop is if the socket object is the server. If it is, a new
client is trying to connect and the accept_new_connection method is called.
Otherwise, the client socket is read. If a null is returned from recv, the peer socket
closed.
In this case, I construct a message and send it to all connected clients, close the
peer socket, and remove the corresponding object from the descriptor list. If the
recv return is not null, a message is available and stored in str. This message is
distributed to all other clients using broadcast_string.
Listing 17. The chat server run method is the core of the chat server
def run( self ):
while 1:
# Await an event on a readable socket descriptor
(sread, swrite, sexc) = select.select( self.descriptors, [], [] )
# Iterate through the tagged read descriptors
for sock in sread:
# Received a connect to the server (listening) socket
if sock == self.srvsock:
self.accept_new_connection()
else:
# Received something on a client socket
str = sock.recv(100)
# Check to see if the peer socket closed
if str == '':
host,port = sock.getpeername()
str = 'Client left %s:%srn' % (host, port)
self.broadcast_string( str, sock )
sock.close
self.descriptors.remove(sock)
else:
host,port = sock.getpeername()
newstr = '[%s:%s] %s' % (host, port, str)
self.broadcast_string( newstr, sock )
Helper methods
The two helper methods in the chat server class provide methods for accepting new
client connections and broadcasting messages to the connected clients.
The accept_new_connection method (see Listing 18) is called when a new client
is detected on the incoming connection queue. The accept method is used to
accept the connection, which returns the new socket object and remote address
information. I immediately add the new socket to the descriptors list, then send a

salutation to the new client welcoming the client to the chat. I create a string
identifying that the client has connected and broadcast this information to the group
using broadcast_string (see Listing 19).
Note that in addition to the string being broadcast, a socket object is also passed.
The reason is that I want to selectively omit some sockets from getting certain
messages. For example, when a client sends a message to the group, the message
goes to the group but not back to itself. When I generate the status message
identifying a new client joining the group, it goes to the group but not the new client.
This task is performed in broadcast_string with the omit_sock argument. This
method walks through the descriptors list and sends the string to all sockets that
are not the server socket and not omit_sock.
Listing 18. Accepting a new client connection on the chat server
def accept_new_connection( self ):
newsock, (remhost, remport) = self.srvsock.accept()
self.descriptors.append( newsock )
newsock.send("You're connected to the Python chatserverrn")
str = 'Client joined %s:%srn' % (remhost, remport)
self.broadcast_string( str, newsock )
Listing 19. Broadcasting a message to the chat group
def broadcast_string( self, str, omit_sock ):
for sock in self.descriptors:
if sock != self.srvsock and sock != omit_sock:
sock.send(str)
print str,
Instantiating a new ChatServer
Now that you've seen the Python chat server (under 50 lines of code), let's see how
to instantiate a new chat server object in Python.
Start the server by creating a new ChatServer object (passing the port number to
be used), then calling the run method to start the server and allow incoming
connections:
Listing 20. Instantiating a new chat server
myServer = ChatServer( 2626 )
myServer.run()
At this point, the server is running and you can connect to it from one or more
clients. You can also chain methods together to simplify this process (as if it needs

to be simpler):
Listing 21. Chaining methods
myServer = ChatServer( 2626 ).run()
which achieves the same result. I'll show the ChatServer class in operation.
Demonstrating the ChatServer
Here's the ChatServer in action. I show the output of the ChatServer (see Listing
22 and the dialog between two clients (see Listing 23 and Listing 24). The
user-entered text appears in bold.
Listing 22. Output from the ChatServer
[plato]$ python pchatsrvr.py
ChatServer started on port 2626
Client joined 127.0.0.1:37993
[127.0.0.1:37994] Hello, is anyone there?
[127.0.0.1:37993] Yes, I'm here.
[127.0.0.1:37993] Client left 127.0.0.1:37993
Listing 23. Output from Chat Client #1
[plato]$ telnet localhost 2626
Trying 127.0.0.1...
Connected to localhost.localdomain (127.0.0.1).
Escape character is '^]'.
You're connected to the Python chatserver
[127.0.0.1:37994] Hello, is anyone there?
Yes, I'm here.
^]
telnet> close
Connection closed.
[plato]$
Listing 24. Output from Chat Client #2
[plato]$ telnet localhost 2626
Trying 127.0.0.1...
Connected to localhost.localdomain (127.0.0.1).
Escape character is '^]'.
You're connected to the Python chatserver
Hello, is anyone there?
[127.0.0.1:37993] Yes, I'm here.
[127.0.0.1:37993] Client left 127.0.0.1:37993
As you see in Listing 22, all dialog between all clients is emitted to stdout, including
client connect and disconnect messages.

Section 6. High-level networking classes
Networking modules
Python includes several specialized modules for application-layer protocols (built on
the standard socket module). The available modules are wide and varied and they
provide module implementations of the Hypertext Transfer Protocol (HTTP), the
Simple Mail Transfer Protocol (SMTP), Internet Message Access Protocol (IMAP)
and Post Office Protocol (POP3), the Network News Transfer Protocol (NNTP),
XML-RPC (remote procedure call), FTP, and many others.
This section demonstrates the modules shown in Table 4.
Table 4. Useful application-layer protocol modules
Module Protocol implemented
httplib HTTP client
smtplib SMTP client
poplib POP3 client
The httplib (HTTP client)
The HTTP client interface can be useful when developing Web robots or other
Internet scraping agents. The Web protocol is request/response in nature over
stream sockets. Python makes it easy to build Web robots through a simple Web
interface.
Listing 25 demonstrates the httplib module. You create a new HTTP client
instance with HTTPConnection, providing the Web site to which you want to
connect. With this new object (httpconn), you can request files with the request
method. Within request, you specify the HTTP GET method (which requests a file
from the server, compared to HEAD which simply retrieves information about the file).
The getresponse method parses the HTTP response header to understand if an
error was returned. If the file was successfully retrieved, the read method on the
new response object returns and prints the text.
Listing 25. Building a simple (non-rendering) HTTP client with httplib

import httplib
httpconn = httplib.HTTPConnection("www-130.ibm.com")
httpconn.request("GET", "/developerworks/index.html")
resp = httpconn.getresponse()
if resp.reason == "OK":
resp_data = resp.read()
print resp_data
httpconn.close()
The smptlib (SMTP client)
SMTP allows you to send e-mail messages to a mail server which can be useful in
networking systems to relay status about the operation of a device. The Python
module for sending e-mail messages is simple and consists of creating an SMTP
object, sending an e-mail message using the sendmail method, then closing the
connection with the quit method.
The example in Listing 26 demonstrates sending a simple e-mail message. The msg
string contains the body of the message (which should include the subject line).
Listing 26. Sending a short e-mail message with smtplib
import smtplib
fromAdrs = 'mtj@mtjones.com'
toAdrs = 'you@mail.com'
msg = 'From: me@mail.comrnTo: you@mail.comrnSubject:HellornHi!rn'
mailClient = smtplib.SMTP('192.168.1.1')
mailClient.sendmail( fromAdrs, toAdrs, msg )
mailClient.quit
The poplib (POP3 client)
POP3 is another useful application-layer protocol for which a module exists within
Python. The POP3 protocol allows you to connect to a mail server and download
new mail, which can be useful for remote commanding -- embedding commands
within the body of an e-mail message. After executing the embedded command, you
can use smptlib to return a response e-mail message to the source.
This demonstration in Listing 27 shows a simple application that connects to a mail
server and emits the subject lines for all pending e-mail for the user.

The poplib is relatively simple but offers several methods for gathering and
managing e-mail at the server. In this example, I create a new POP3 object with the
POP3 method, specifying the mail server. The user and pass_ methods
authenticate the application to the server; the stat method returns the number of
messages waiting for the user and the total number of bytes taken up by all
messages.
Next, I loop through each available message and use the retr method to grab the
next e-mail message. This method returns a list of the form:
(response, ['line, ...], octets)
where response is the POP3 response for the particular message, the line list
represents the individual lines of the e-mail message, and the final element, octets,
is the number of bytes for the e-mail message. The inner loop simply iterates over
the second element ([1]) of the list which is the list of the e-mail message body. For
each line, I test whether 'Subject:' is present; if so, I print this line.
After all e-mail messages have been checked, a call to the quit method ends the
POP3 session.
Instead of using the retr method, you could also use the top method to extract the
header for the e-mail message. This step would be faster and minimize the amount
of data transferred to this client.
Listing 27. Retrieving e-mail messages from a POP3 mail server and emitting
the subject line
import poplib
import re
popClient = poplib.POP3('192.168.1.1')
popClient.user('user')
popClient.pass_('password')
numMsgs, mboxSize = popClient.stat()
print "Number of messages ", numMsgs
print "Mailbox size", mboxSize
print
for id in range (numMsgs):
for mail in popClient.retr(id+1)[1]:
if re.search( 'Subject:', mail ):
print mail
print
popClient.quit()

Section 7. Summary
Summary
This tutorial reviewed the basics of the sockets API and showed how to build
networking applications in Python. The standard sockets module was introduced as
a way to build both client and server applications, as well as the SocketServer
module which simplifies the construction of simple socket servers. I presented a
simple chat server implemented in Python that offered support for a scalable number
of clients using the select method. In closing, I previewed some of Python's
high-level networking classes that simplify the development of applications requiring
application-layer networking protocols.
Python is an interesting and useful language that is well worth your time to learn.
The Resources section provides more information to help you dig in further.

Resources
Learn
• Learn more about sockets programming in these developerWorks tutorials
Programming Linux Sockets, Part 1 and Programming Linux Sockets, Part 2.
• Discover network programming in a variety of languages (including Python) in
BSD Sockets Programming from a Multi-Language Perspective by M. Tim Jones
(Charles River Media, 2003).
• In "Pyrex extends and speeds Python apps" (developerWorks, January 2005),
David Mertz shows you how to speed up Python.
• David's Charming Python series provides a great look at Python and its vast
capabilities.
• Read an interesting introduction to Python from Dave Kuhlman.
• The Internet Protocols and Support section of the online Python documentation
demonstrates the various networking modules.
• Find more resources for Linux developers in the developerWorks Linux zone.
Get products and technologies
• Download the latest version of Python from the official Python Web site.
• You can use the Python language within Apache by using mod_python.
• Order the SEK for Linux, a two-DVD set containing the latest IBM trial software
for Linux from DB2®, Lotus®, Rational®, Tivoli®, and WebSphere®.
• Build your next development project on Linux with IBM trial software, available for
download directly from developerWorks.
Discuss
• Get involved in the developerWorks community by participating in
developerWorks blogs.
About the author
M. Tim Jones
M. Tim Jones is a senior principal software engineer with Emulex Corp. in Longmont,
Colorado, where he architects and designs networking and storage products. Tim's
design activities have ranged from real-time kernels for communication satellites to

networking protocols and embedded firmware. He is the author of many articles on
subjects from artificial intelligence (AI) to application-layer protocol development. He
has also the author of AI Application Programming (now in its second edition),
GNU/Linux Application Programming, BSD Sockets Programming from a
Multilanguage Perspective, and TCP/IP Application Layer Protocols for Embedded
Systems (all through Charles River Media).

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